1. Field of Invention
This invention relates to soft and flexible electrical heaters, and particularly to heating elements, including conductive threads/fibers.
2. Description of the Prior Art
Heating elements have extremely wide applications in household items, construction, industrial processes, etc. Their physical characteristics, such as thickness, shape, size, strength, flexibility and other characteristics affect their usability in various applications.
Numerous types of thin and flexible heating elements have been proposed. For example, U.S. Pat. No. 4,983,814 proposed by Ohgushi is based on a proprietary electroconductive fibrous heating element produced by coating an electrically nonconductive core fiber with electroconductive polyurethane resin containing the carbonatious particles dispersed therein. Ohgushi""s manufacturing process appears to be complex; it utilizes solvents, cyanates and other toxic substances. The resulting heating element has a temperature limit of 100xc2x0 C. and results in a pliable but not soft heating element. In addition, polyurethane, used in Ohgushi""s invention, when heated to high temperature, will decompose, releasing very toxic substances, such as products of isocyanide. As a consequence, such heating element must be hermetically sealed in order to prevent human exposure to toxic offgassing. Ohgushi claims temperature self-limiting quality for his invention, however xe2x80x9cactivationxe2x80x9d of this feature results in the destruction of the heater. He proposes the use of the low melting point non-conductive polymer core for his conductive fabric-heating element, which should melt prior to melting of the conductive layer, which uses the polyurethane binder with the melting point of 100xc2x0 C. Thus, the heating element of Ohgushi""s invention operates as Thermal Cut Off (TCO) unit, having low temperature of self-destruction, which limits its application.
U.S. Pat. No. 5,861,610 to John Weiss describes the heating wire, which is formed with a first conductor for heat generation and a second conductor for sensing. The first conductor and a second conductor are wound as coaxial spirals with an insulation material electrically isolating two conductors. The two spirals are counter-wound with respect to one another to insure that the second turns cross, albeit on separate planes, several times per inch. The described construction results in a cable, which has to be insulated twice: first, over the heating cable and second, over the sensor cable. The double insulation makes the heating element very thick, stiff and heavy, which would be uncomfortable for users of soft and flexible products such as blankets and pads. The described cable construction cannot provide large heat radiating area per length of the heater, as it would be possible with a strip or sheet type of the heating element. The termination with electrical connectors is very complicated because of stripping two adjacent layers of insulation. In addition, in the event of overheating of a very small surface area of the blanket or pad (for example several square inches), the sensor may fail to sense a very low change in the total electrical resistance of the long heating element due to operating resistance tolerance of the heating cable. In addition, such heating cable does not have Thermal-Cut-Off (TCO) capabilities in the event of malfunction of the controller.
Another prior art example is U.S. Pat. No. 4,309,596 to George C. Crowley, describing a flexible self-limiting heating cable, which comprises two metal conductor wires separated by a positive temperature coefficient (PTC) material. Said heating metal wires are disposed on textile fiber core made of strands of nonconductive fibers coated with conductive carbon. This method has the following disadvantages: The textile fiber core serves only for aligning of metal wire conductors and strengthening of the heating cable. In the event of abnormal overheating, or fire, the destruction of textile fiber core will only increase the possibility of short circuit and fire hazard. In addition, the metal conductor wires have very high melting temperature; therefore they cannot provide TSL or TCO safety functions.
Thrash (U.S. Pat. No. 5,801,914) describes an electrical safety circuit that utilizes a sacrificial fuse filament. Such sacrificial filament is connected to a separate switching circuit, which terminates electrical continuity of PTC heating element in the event of fire hazard. The main disadvantages of this design are (a) the switching circuit deactivates power only after arcing/fire has already started and burned the sensor fiber filament; (b) utilizing of two separate circuits: one for heating, another for fire sensing, significantly increases cost of the products, and (c) addition of sensing sacrificial filament enlarges overall thickness of conventional PTC cable, which originally suffered from excessive stiffniess and bulkiness.
Another prior art example is U.S. Pat. No. 4,969,840 to Li Hidehiro at al, describing an electrical connector for flexible flat cable. Such electrical connector comprises plastic housing having plurality of terminals arranged therein and a removable connector cover mounted on the connector housing. When the flexible flat cable is inserted and the connector cover is pushed into the connector housing, the lock hole of the cable becomes engaged with the projection of the lock plate. The design of the connector has the following disadvantages: (a) the connector cannot be applied for soft cable having unstable shape such as woven textile strips; (b) the housing of connector is complicated and requires cable with special hole in the center, (c) the connectors, cannot provide reliable electrical connection with soft conductors such as electrically conductive textile threads.
The present invention seeks to alleviate the drawbacks of the prior art and describes the fabrication of a heating element, comprising temperature self-limiting (TSL) electrically conductive threads/fibers, which is economical to manufacture; does not pose environmental hazards; results in a soft, flexible, strong, thin, and light-weight heating element core, suitable for even small and complex assemblies, such as handware. A preferred embodiment of the invention consists of utilizing conductive textile threads having Thermal Cut Off (TCO) and/or TSL functions to prevent overheating and fire hazard. Unique methods of electrical termination of conductive textile heating element are also described below.
The first objective of the invention is to provide a significantly safe and reliable heating element which can function properly after it has been subjected to sharp folding, kinks, small perforations, punctures or crushing, thereby solving problems associated with conventional flexible heating metal wires. In order to achieve the first objective, the electric heating element of the present invention is comprised of electrically conductive textile heating threads/fibers. The conductive heating threads/fibers may comprise carbon or metal microfibers, or textile threads coated/impregnated with, at least one of the following electrically conductive materials: metal, carbon/graphite, carbides, conductive ink, metal sulfides, metal zeolites, metal oxides, metal filled polymers, metal coated nonconductive particles or their combination. The conductive heating threads/fibers possess the following characteristics: (a) high strength; (b) high strength-to-weight ratio; (c) softness, (d) flexibility. The heating element core described in this invention has a shape of electrically conductive strips, sleeves, sheets, ropes or cables, which radiate a controlled heat over the entire heating core surface.
The second objective of the invention is to provide a high level of safety, minimizing the possibility of fire hazard. In order to achieve the second objective: (A) the bus conductors of heating elements may comprise temperature sensitive conductive textile threads/fibers having melting point from 120xc2x0 C. to 350xc2x0 C. The melting of the conductive threads/fibers results in complete breaking of electrical continuity in the heating system, or portion thereof, in the event of abnormal local overheating; (B) the heating means comprises electrically conductive threads, which have melting point from 120xc2x0 C. to 350xc2x0 C.; and (C) at least bus conductors or heating means may comprise temperature self-limiting (TSL) conductive textile threads. If abnormal overheating occurs in the heating system, and the temperature elevates slowly, reaching the melting point, the TSL threads/fibers, described in this invention, increase their electrical resistance, thus decreasing the outcome power.
Therefore, the proposed bus conductors and/or conductive textile heating means can safely operate both as high temperature TCO (Thermal-Cut-Off) and as TSL devices. The above described TCO and TSL safety functions activate mainly in the event, when normal temperature regulators, such as thermostats or power controllers do not provide sufficient thermal regulation, or malfunction in the heating products.
The third objective of the invention is to provide redundancy of the electrical circuit if necessary. In order to achieve the third objective (A) the heating element core may comprise electrically conductive threads or metal wires bridging the electrical continuity between heating electroconductive threads and (B) the bus conductors, which contain metal wires, may comprise electrically conductive threads, bridging electrical continuity between said metal wires.
The forth objective of the invention is to provide reliable mechanical and electrical connection between heating conductive textile threads/fibers of the heating element and metal wire conductors during electrical termination of the heating products. In order to achieve the forth objective, at least one of the following methods may be applied:
(A) joining of electrically conductive heating threads with lead wires in one bundle, followed by winding, preferably, by thin metal wires or electrically conductive textile threads around such bundle. Nonconductive threads or polymer monofilaments may also be utilized for such winding. A metal crimp, auto splice and/or plastic tie can be placed on the top of such bundle to insure reliable mechanical and electrical connection.
(B) utilizing of telescoping crimping, which comprises steps of: (i) placing of electrically conductive threads between at least two ductile materials, provided that at least one of them has electrically conductive surface, which is in contact with electrically conductive threads, and (ii) crimping of the whole terminal assembly.
The fifth objective of the invention is to provide quick disconnection of the heating element from the electrical power source in order to prevent tripping and injuring of consumers who utilize a heating device according to the preferred embodiment of this invention. Such quick electric power disconnection is extremely important for products utilized in xe2x80x9chigh trafficxe2x80x9d zones, such as heated area rugs and mats, heated chairs and sofas, space radiant heaters and other heating devices requiring an extended electrical cord to reach a power outlet. In order to achieve the fifth objective, the male connector box, having trapezoidal cross section of the cavity is utilized in order to quickly disconnect electrical continuity in the heating element assembly.
The present invention comprises a heating element, containing soft, strong and light electrically conductive textile acting as temperature self-limiting heating media. The heating element is highly resistant to punctures, small perforations, sharp folding and crushing. It can be manufactured in various shapes and sizes, and it can be designed for a wide range of parameters, such as input voltage, desired temperature range, desired power density, type of current (AC and DC) and method of electrical connection (parallel and in series).